1. Technical Field
This invention generally relates to photodetectors and, more particularly, to a traveling wave photodetector having a plurality of metal-semiconductor-metal electrodes formed thereon with a tapering electrode finger width and gap configuration.
2. Discussion
Fiber optic links are employed in a myriad of modem applications. High frequency optical detectors are one of the primary components that dictate the performance of a fiber optic link. In order to redue the insertion loss of radio frequencies, increase the spurious free dynaic range, and increase the signal-to-noise ratio of the link, the photodetector must accommodate high optical powers. The performance of photodetectors for microwave and millimeter wave applications is characterized by their bandwidth, optical-to-electrical conversion efficiency, and maximum output photocurrent. The maximum output photocurrent can be specified at the point of catastrophic failure or at 1 dB compression, whichever occurs first.
Several approaches have been proposed in the prior art to increase these figures of merit including traveling wave photodetectors, see, K. S. Giboney et. al., "Traveling Wave Photodetectors," IEEE Photonics Technology Letters, Volume 4, pages 1363-1365 (1992) and velocity-matched distributed photodetectors, see, L. Y. Lin et. al., "High-Power High-Speed Photodetectors--Design, Analysis, and Experimental Demonstration," IEEE Transactions on Microwave Theory and Techniques, Volume 45, pages 1320-1330 (1997). In these approaches, the photodetector comprises an optical waveguide and a microwave transmission line. According to this configuration, there is a co-propagation of an optical wave and an induced microwave signal which, when properly matched, travel in-phase down the length of the detector.
By matching the group velocity between the microwave and optical signals, the photodetector can be made electrically long while maintaining bandwidths in excess of a few hundred GHz. The excessively long length of the photodetector provides two key benefits. First, the optical-to-electrical conversion efficiency can be made to approach the quantum limit. Second, the absorbed optical power density can be kept small thereby circumventing saturation and/or catastrophic failure so that the maximum output photocurrent can be increased. Photocurrents in excess of 50 mA have been previously reported by Lin et. al.
Referring to FIG. 2, one of the more promising variations of these velocity-matched distributed photodetectors is illustrated. The photodetector consists of an array of Metal-Semiconductor-Metal (MSM) photodiodes 20 serially connected by an integrated passive semiconductor optical waveguide 22. The electrode structure array consists of the interdigitated electrodes 24 of the photodiode 20 connected to a Coplanar Strips (CPS) transmission line 26.
Referring to FIG. 7, refers to a close-up view of a single MSM waveguide photodetector for the array. As illustrated, the photodiode 202 is grown on a semi-insulating InP substrate 210. In operation, intensity modulated light (illustrated as the large block arrow) propagates down the optical waveguide 204 and couples into the electrodes 206 via evanescent coupling. As the intensity modulated light is absorbed by a thin semiconducting absorbing layer 30 (FIG. 4), a microwave signal is generated at the frequency of the optical intensity modulation and propagates down the transmission line 208. The promise of this variation is due to the fact that photodiodes with bandwidths in excess of 300 GHz have been demonstrated, see, S. Y. Chou et. al., "Nanoscale Terahertz Metal-Semiconductor-Metal Photodetectors," EEEE J. Quantum Electron, Volume 28, Number 10, pages 2358-2368 (1992). In addition, Metal-Semiconductor-Metal structures have superior microwave transmission loss characteristics compared to p-i-n photodetectors due to the lack of heavily doped p and n regions.
Referring now to FIG. 8, a cross-sectional view of the conventional photodiode 202 is illustrated. As can be seen, according to the prior art, the width w of each electrode finger 212 and the gap g between the fingers 212 remains constant down the entire length of the photodiode 202. Since optical power decays exponentially down the length of the photodiode 202, the amount of photocurrent that the fingers 212 are required to handle also decays exponentially. A consequence of this is that finger 214 will fail first and the subsequent fingers 212 down the length of the photodiode 202 are all underutilized.
In addition, the frequency response of conventional detectors such as the photodiode 202 is limited by the time it takes for the optically generated electron-hole pairs to travel to a finger 212. Typical electric field lines are shown in FIG. 8 and are representative of alternately charged metal lines. As the electron-hole pairs are generated in the absorption layer 216 of the photodiode 202, the carriers are separated from one another by the electric field and are accelerated towards oppositely charged fingers 212. The strength of the electric field under the fingers 212 and the distance the carriers must travel limits the frequency response.
The 1 dB compression point of the photocurrent is also dependent on the electric field strength in the absorbing layer of the fingers 212. In FIG. 8, it can be seen that directly underneath the center of each of the fingers 212 are regions of low electric field strength. Carriers that are generated in these regions experience longer transit-times thereby degrading the frequency response of the photodiode 202 and reducing the 1 dB compression point of the photocurrent.
From the above, it can be appreciated that the prior investigations of distributed detectors have focused on the appropriate periodic loading of the Metal-Semiconductor-Metal photodiodes 202 on the transmission line 208 to achieve the velocity match between the optical wave and the induced microwave. Although there has been a significant amount of work on top-illuminated Metal-Semiconductor-Metal photodiodes, very little work has addressed the individual Metal-Semiconductor-Metal photodiodes in this waveguide configuration. To date, all implementations of the Metal-Semiconductor-Metal interdigitated electrodes have consisted of uniformly spaced electrode fingers and electrode finger widths.
To reduce the regions of low electric field strength in the absorbing layer, it is desirable to maintain a high aspect ratio of gap to finger width, r=g/w. To maintain short carrier transit-times (large bandwidths), the gap between the fingers 212 must also be kept small. As a result, to obtain a high aspect ratio, the width of the fingers 212 must become increasingly small. Unfortunately, as the width of the fingers 212 is decreased, the photocurrent density in each finger 212 increases causing the photodiode 212 to fail at lower output photocurrents.
In view of the foregoing, it would be desirable to provide an electrode design to significantly improve the performance of the waveguide coupled metal-semiconductor-metal photodiode. More particularly, it would be desirable to provide an electrode design that will improve the inherent trade-off between electric field uniformity in the absorbing layer and photocurrent density in the metal-semiconductor-metal electrode fingers.